A UK request that a former KGB agent be extradited over the murder of Alexander Litvinenko is "pure foolishness", Russian President Vladimir Putin says.
Andrei Lugovoi has denied the charges
Russia's constitution did not permit it to hand over citizens and British prosecutors' competence was in doubt if they had not known that, Mr Putin said.
He said if the UK sent enough evidence a trial could be held in Russia.
The UK wants to charge Andrei Lugovoi, who denies involvement, with murdering Mr Litvinenko, 43, in London in 2006.
He died in November after exposure to the radioactive isotope polonium-210.
Mr Litvinenko, who was granted political asylum in the UK in 2000, was a former KGB agent himself and a critic of Mr Putin.
The UK's director of public prosecutions has recommended that Mr Lugovoi be tried for murder by "deliberate poisoning" and a formal extradition request has been handed over to the authorities in Moscow.
KEY EVENTS IN CASE
1 November 2006: Alexander Litvinenko meets Andrei Lugovoi and another Russian at a London hotel
23 November 2006: Litvinenko dies in a London hospital
24 November 2006: A Litvinenko statement accuses Russian President Vladimir Putin of involvement in his death. Experts say Litvinenko was poisoned
6 December 2006: UK police say they are treating the death as murder
22 May 2007: Lugovoi should be charged with Litvinenko's murder, British prosecutors say
28 May 2007: UK makes formal request for Lugovoi's extradition from Russia
The request has been made under the 1957 Council of Europe European Convention on Extradition, of which Russia is a signatory. However, Russia does have the right, under Article 6, to refuse to extradite one of its nationals.
Speaking to journalists ahead of his attendance at the G8 summit in Germany, Mr Putin reiterated the fact that Russia can prosecute a citizen for a crime in a foreign country.
However, he said the UK had not yet supplied it with sufficient details of the case against Mr Lugovoi, who met Mr Litvinenko on the day he fell ill.
"Rather than simply demand Lugovoi's extradition, they should send enough evidence for the case to be taken to court," he said.
"We'll do it in Russia, and any person found guilty of causing Litvinenko's death will be convicted."
Mr Litvinenko's widow, Marina, has dismissed Mr Lugovoi's claims that British secret services had a part in the death.
She said her husband's case was different from anything that had happened before and Russia should reconsider its law over extraditions.